If Specialp3 is correct though then only a 260Z glass will fit his 240. And the 260 was a one-year only model in the USA.
But if there was a change in the bolting pattern for whatever reason then I suspect it was done part-way through the 260 & 280Z production years.
Toughened glass can't be altered, but maybe the mounting bracket can be?
Specialp3, have you touched base with the NZ Datsun Z Club? NZ has a variety of 240/260/280/2+2 Zeds, some sold NZ-new, but many others are used car imports from Australia, the UK, Nth America, Japan, Asia, wherever, often brought in by returning expats. It shouldn't be impossible to find a blue-tint door glass, I could be wrong but my impression when I was a club member there - many years ago now - was that clear glass was less common in club cars than blue-tint. Worth a try, or check out TradeMe.com the NZ equivalent to eBay.